Effect Of Sulfur Substitution Research Articles

Quasi-intrinsic thiobase derivatives as potential targeted photosensitizers in two-photon photodynamic therapy

In this study, a set of potential quasi-intrinsic photosensitizers for two-photon photodynamic therapy (PDT) are proposed based on the unnatural 2-amino-8-(1′-β-ᴅ-2′-deoxyribofuranosyl)-imidazo[1,2-ɑ]-1,3,5-triazin-4(8H)-one (P), which is paired with the 6-amino-5-nitro-3-(1′-β-ᴅ-2′-deoxyribofuranosyl)-2(1H)-pyridone (Z) and can specifically recognize breast and liver cancer cells. Herein, the effects of sulfur substitution and electron-donating/electron-withdrawing groups on the photophysical properties in aqueous solution are systematically investigated. The one- and two-photon absorption spectra evidence that the modifications could result in red-shifted absorption wavelength and large two-photon absorption cross-section, which contributes to selective excitation and provides effective PDT for deep-seated tissues. To ensure the efficient triplet state population, the singlet–triplet energy gaps and spin–orbit coupling constants were examined, which is responsible for a rapid intersystem crossing rate. Furthermore, these thiobase derivatives are characterized by the long-lived T1 state and the large energy gap for radiationless transition to ensure the generation of cytotoxic singlet oxygen.

Oxysulfide perovskites: reduction of the electronic band gap of RbTaO3 by sulfur substitution

In this study, the effects of sulfur substitution on the structural, mechanical, electronic, optical, and thermodynamic properties of RbTaO3−x S x have been investigated using the WIEN2k code in the framework of density functional theory (DFT). The cubic phase of RbTaO3 transforms to tetragonal for RbTaO2S and RbTaOS2; the latter transforms again to a cubic phase with added sulfur for RbTaS3. The results show that substituting S for O anions in RbTaO3 effectively decreased the band gap [calculated using TB-mBJ functional] from 2.717 eV to 1.438 eV, 0.286 eV, and 0.103 eV for the RbTaO3, RbTaO2S, RbTaOS2, and RbTaS3 compounds, respectively. The optical constants such as dielectric constant, refractive index, absorption coefficient, photoconductivity, reflectivity, and loss function have been calculated and analyzed. The elastic constants and moduli and their anisotropic nature were also investigated. Finally, the Debye temperature, thermal conductivity, melting temperature, specific capacities, and thermal expansion coefficients were computed and analyzed using established formalisms. The reduced band gap (1.438 eV) and high absorption coefficient (∼106 cm−1) of RbTaO2S, make it suitable for solar cell applications and for other visible light devices. Both ordered structure and super-cell have been used to cross-check results for RbTaO2S and RbTaOS2 compositions.

A DFT study on Electronic Excitations, Charge Transfer and NLO Properties of Visible Absorbing Squaraine and Thiosquaraine dyes

Density functional calculations have been performed to see the effect of sulfur substitution in place of oxygen at central four membered acceptor squarate ring on electronic excitations, charge transfer and second order non-linear optical properties in visible absorbing squaraines (SQ). Molecules oxy-thiosquaraines (OSQ) and thiosquaraines (SSQ) have showed red shift in absorption as compared to their corresponding SQ molecules. The lowest five electronic excitations for all the molecules have been calculated by using TDDFT method. Further, effect of electron donating and electron withdrawing groups on absorption maxima have been studied. The large red shifts in case of electron withdrawing group within the same series of molecules (SQ, OSQ and SSQ) are due to destabilization of HOMO and stabilization of LUMO levels. Charge transfers in these molecules are reported by using VMOdes. Second hyperpolarizabilities (áγñ) for these molecules are calculated by SOS method. This study may helpful in synthesizing new C-N bonding SQ, OSQ and SSQ dyes which are further useful in NLO applications.

Effect of Sulfur Substitution on Charge Transport Ability of Benzopyrazine-Fused Tetracene Derivatives: A Theoretical Study

Effect of Sulfur Substitution on Charge Transport Ability of Benzopyrazine-Fused Tetracene Derivatives: A Theoretical Study

Effects of Sulfur Substitution on Chemical Bonding Nature and Electrochemical Performance of Layered LiMn0.9Cr0.1O2-xSx

Sulfur-substituted LiMn 0.9 Cr 0.1 O 2-x S x (0≤x≤0.1) layered oxides have been prepared by solid state reaction under inert atmosphere. From powder X-ray diffraction analyses, all the present lithium manganates were found to be crystallized with monoclinic-layered structure. Electrochemical measurements clearly demonstrated that, in comparison with the pristine LiMn 0.9 Cr 0.1 O 2 , the sulfur-substituted derivatives exhibit smaller discharge capacities for the entire cycle range but the recovery of discharge capacity after the initial several cycles becomes faster upon sulfur substitution. The effect of the sulfur substitution on the chemical bonding nature of LiMn 0.9 Cr 0.1 O 2-x S x has been investigated using X-ray absorption spectroscopic (XAS) analyses at Mn and Cr K-edges. According to Mn K-edge XAS results, the trivalent oxidation state of manganese ion remains unchanged before and after the substitution whereas the local structure around manganese ions becomes more distorted with increasing the substitution rate of sulfur. On the other hand, the replacement of oxygen with sulfur has negligible influence on the local atomic arrangement around chromium ions, which is surely due to the high octahedral stabilization energy of Cr +III ions. Based on the present experimental findings, we have suggested that the decrease of discharge capacity upon sulfur substitution is ascribable to the enhanced structural distortion of MnO 6 octahedra and/or to the formation of covalent Li-S bonds, and the accompanying improvement of cyclability would be related to the depression of Mn migration and/or to the pillaring effect of larger sulfur anion.

Vibrational frequencies and structure of 2-thiouracil by Hartree–Fock, post-Hartree–Fock and density functional methods

Vibrational study of the biomolecule 2-thiouracil was carried out. Ab initio and density functional calculations were performed to assign the experimental spectra. A comparison with the uracil molecule was made, and specific scale factors were deduced and employed in the predicted frequencies of 2-thiouracil. Several scaling procedures were used. The geometry structure of the molecule was determined. The effect of sulfur substitution at C2 position in the uracil molecule, on the N1H and N3H frequencies and intensities reflects changes in proton donor abilities of these groups. Calculations with the 6-31 G** basis set with HF and DFT methods appear in general to be useful for interpretation of the general features of the IR and Raman spectra of the molecule. Using specific scale factors a very small error was obtained. The use of these specific scale factors resolve and correct some of the controversial assignments in the literature.

Specific metal-ion binding sites in a model of the P4-P6 triple-helical domain of a group I intron.

Divalent metal ions play a crucial role in RNA structure and catalysis. Phosphorothioate substitution and manganese rescue experiments can reveal phosphate oxygens interacting specifically with magnesium ions essential for structure and/or activity. In this study, phosphorothioate interference experiments in combination with structural sensitive circular dichroism spectroscopy have been used to probe molecular interactions underlying an important RNA structural motif. We have studied a synthetic model of the P4-P6 triple-helical domain in the bacteriophage T4 nrdB group I intron, which has a core sequence analogous to the Tetrahymena ribozyme. Rp and Sp sulfur substitutions were introduced into two adjacent nucleotides positioned at the 3' end of helix P6 (U452) and in the joining region J6/7 (U453). The effects of sulfur substitution on triple helix formation in the presence of different ratios of magnesium and manganese were studied by the use of difference circular dichroism spectroscopy. The results show that the pro-Sp oxygen of U452 acts as a ligand for a structurally important magnesium ion, whereas no such effect is seen for the pro-Rp oxygen of U452. The importance of the pro-Rp and pro-Sp oxygens of U453 is less clear, because addition of manganese could not significantly restore the triple-helical interactions within the isolated substituted model systems. The interpretation is that U453 is so sensitive to structural disturbance that any change at this position hinders the proper formation of the triple helix.

The effects of sulfur substitution in chiral amino thiols on the enantioselective addition of organozinc reagents to aldehydes: a novel method for estimation of free energies of dimerization in monomer–dimer equilibria

Differences between the thiol ligand 1 and the corresponding alcohol ligand 2 were observed in the catalytic asymmetric alkylation of benzaldehyde with diethylzinc. The thiol ligand 1 was superior for reaction rate, enantioselectivity and asymmetric amplification. The effects of chiral amino thiols are discussed and compared with the results of chiral amino alcohol counterparts. The quantitative and thermodynamic aspects of the monomer–dimer equilibria involved in thiazazincolidine or oxazazincolidine catalysts have also been studied on the basis of colligative properties.

Effect of Sulfur Substitution on the Floral Odor of Tetrahydropyranyl and Tetrahydrofuranyl Ethers

Sulfur-containing analogues of tetrahydropyranyl and tetrahydrofuranyl derivatives of selected alcohols and phenols have been prepared and their odor properties evaluated. The compounds include derivatives in which either one or both of the oxygen atoms had been replaced with sulfur. Derivatives with two sulfur atoms do not exhibit pleasant odors; those with an endocyclic oxygen and an exocyclic sulfur generally exhibit green notes. The floral character is found only in a few derivatives with an endocyclic sulfur and an exocyclic oxygen

Phospholipids chiral at phosphorus. Characterization of the sub-gel phase of thiophosphatidylcholines by use of X-ray diffraction, phosphorus-31 nuclear magnetic resonance, and Fourier transform infrared spectroscopy.

A recent study using differential scanning calorimetry (DSC) showed that the thermotropic phase behavior of 1,2-dipalmitoyl-sn-glycero-3-thiophosphocholine (DPPsC) is sensitive to the configuration at phosphorus and that the Rp isomer displayed only a broad transition at 45.6 degrees C [Wisner, D. A., Rosario-Jansen, T., & Tsai, M.-D. (1986) J. Am. Chem. Soc. 108, 8064-8068]. We have employed X-ray diffraction, 31P NMR, and Fourier transform infrared (FT-IR) spectroscopy to characterize various phases of the isomers of DPPsC, to compare the structural differences between 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and isomers of DPPsC, and to identify structural factors responsible for the unique behavior of the RP isomer. The results from all three techniques support the previous proposal based on DSC studies that (SP)- and (RP + SP)-DPPsC undergo a subtransition, a pretransition, and a main transition analogous to those of DPPC, while (RP)-DPPsC is quite stable at the subgel phase and undergoes a direct subgel----liquid-crystalline transition at 46 degrees C. Quantitative differences between DPPC and DPPsC (i.e., the effect of sulfur substitution rather than the configurational effect) in the subgel phase have also been observed in the chain spacing, the motional averaging, and the factor group splitting (revealed by X-ray diffraction, 31P NMR, and FT-IR, respectively). In particular, DPPsC isomers are motionally rigid and show enhanced factor group splitting in the subgel phase. These results suggest that DPPsC is packed in different subcells relative to DPPC in the subgel phase.(ABSTRACT TRUNCATED AT 250 WORDS)

Atomic dipole approximation and energies of interactions between purine and pyrimidine bases. I. Electrostatic interactions of adenine with uracil, thymine, thiouracils, dihydrouracil, and 5‐fluorouracil

AbstractThe atomic dipole approximation has been employed to calculate the energies of electrostatic interaction between adenine and various pyrimidine derivatives. Minima of the interaction energy for various planar configurations were determined. Inclusion of the “monopole–dipole” and “dipole–dipole” terms in the multipole expansion improves considerably the agreement with experimental data. The effect of sulfur substitution has been investigated in detail. Formation of NH…︁S hydrogen bonds is less favorable than of NH…︁O bonds, due largely to the lower atomic dipole of the sulfur atom resulting from the shift of the π‐electron charge toward the neighboring carbon. The results are relevant to the interactions of thiouracils in nucleic acids.